Technical Abstract:
Butanol is the major product of acetone-butanol-ethanol (ABE; ratio 3:6:1) fermentation. It can be produced from various carbohydrates such as glucose, corn, molasses, and whey permeate (a by-product of the dairy industry) using microbial strains such as Clostridium beijerinckii and/or C. acetobutylicum in batch reactors. This chemical can be used as a feedstock or biofuel and contains more energy than ethanol on a weight basis. The maximum concentration of ABE in a batch reactor is limited to 20-30 g/L due to end product inhibition. At that concentration, growth and fermentation of the microbial strain are completely arrested. In an attempt to commercialize this fermentation, economic studies were performed on the use of agricultural residues as substrates in addition to the application of process technology and energy efficient recovery from the fermentation broth. As a result of these studies, we investigated the use of wheat straw, corn stover, barley straw, and switchgrass as economical substrates. The cost of these substrates has been projected to be $24-60 per ton. Unfortunately, fermentation of some of these substrates (barley straw, corn stover, and switchgrass) is difficult due to the generation of inhibitors during pretreatment and hydrolysis. We successfully removed inhibitors from barley straw and corn stover hydrolyzates by overliming them and fermented the detoxified sugar mixture (hexoses and pentoses) successfully. We were also successful in developing an integrated process for the production of butanol from wheat straw. In this process, hydrolysis, fermentation, and ABE recovery were combined. The new process was called simultaneous saccharification, fermentation, and recovery (SSFR). In such an integrated process, product (ABE) concentration in the reactor was kept below 10 g/L (to avoid product inhibition) while in the recovered stream, product level reached over 70 g/L. We also developed butanol producing lactic acid bacterial (LAB) strains such as Lactococcus lactis and Lactobacillus buchneri. Some of the LAB strains are butanol tolerant and can grow in up to 30 g/L butanol solution. The results of these investigations will be presented.